Cell adhesion plays an important role in sustaining life of multicellular organisms. Cell adhesions of multicellular organisms are classified into cell-extracellular matrix (hereinafter abbreviated as “ECM”) adhesion and cell-cell adhesion. It has been elucidated that cell-ECM adhesion is mediated by integrins and cell-cell adhesion is mediated by cadherins, claudins and nectins.
Transmembrane adhesion proteins, such as integrins, constitute cell-ECM adhesions. Integrin forms heterodimer of α and β chains. At least 18 types of α chain, 8 types of β chain and 24 types of αβ heterodimer have been identified and confirmed so far. Each type of integrin recognizes a specific ligand. Transmembrane adhesion proteins including integrins relate to, in addition to cell adhesions, intracellular signal transductions from ECM into a cell and regulation of proliferation, mobility and differentiation (F. G. Giancotti, et. al., Science, 285, 1028-1032, 1999).
Many proteins are known as ECM proteins which are classified into collagens (such as type I-XIX), non-collagenous glycoproteins (such as osteopontin (OPN), vitronectin, fibronectin, von Willebrand Factor, laminin, tenascin, fibrinogen, thrombospondin), elastins and proteoglycans. These ECM proteins bind to corresponding integrins and activate intracellular signal transduction pathways to regulate cytoskeletal organization, mobility, proliferation, differentiation, and the like. ECM protein-bound integrin regulates these signal activating pathways by transmitting specific signals depending on the type of ECM protein. The RGD sequence is commonly observed in cell adhesion region of many ECM proteins and exhibits various functions by binding to integrins. The RGD sequence of ECM proteins has been viewed as a possible target for drugs, and a number of small molecule compounds and artificial peptides have been provided.
Some types of integrins such as α3β1 integrin, α5β1 integrin, α8β1 integrin, αvβ1 integrin, αvβ3 integrin, αvβ5 integrin, αvβ6 integrin, αvβ8 integrin have been known to bind to the RGD sequence. The interaction between α5β1 integrin and its specific ligand fibronectin has inspired investigations into the mechanisms of integrin mediated signal transduction. Such investigations show that α5β1 integrin regulates not only cell adhesion and cell mobility, but also cell differentiation and cell mortality. (S. M. Frisch et al., Curr. Opin. Cell Biol., 9, 701-706, 1997). It has also been shown that α5β1 integrin is highly expressed on tumor cells and relates to malignant alteration of cancer. Each integrin mediated signal differs depending on binding ECM proteins, for example, stimulation by growth factor activates growth of fibronectin-bound endothelial cells, but inhibits growth of laminin-1 bound endothelial cells. Also, the signal transmitted from laminin-10/11 to α3β1 integrin is different from the signal transmitted from fibronectin to α5β1 integrin, and significantly enhances mobility of cancerous cells (J. Gu et al., J. Biol. Chem., 276, 27090-27097, 2001) and significantly avoids apoptosis by blood starvation (J. Gu et al., J. Biol. Chem., 277, 19922-19928, 2002). High expression of RGD sequence binding αv integrins has been observed in the osteoclastic cells and neovascular, and inhibition of the RGD sequence and the αv integrins has been viewed as a target for a therapeutic drug for osteoporosis and cancer. It has been indicated that α5β1 integrin is highly expressed on tumor cells and relates to malignant alteration of cancer. From these findings, anti-α5β1 integrin antibody (Volocimab), anti-α4 integrin antibody (Natalizumab), and anti-αvβ3 integrin antibody (Vitaxin) have been developed as antagonistic anti-integrin antibody drugs which inhibit interaction between integrin and ECM protein.
Meanwhile, some ECM proteins such as collagen, osteopontin (OPN), vitronectin, fibronectin, von Willebrand Factor, laminin, tenascin, fibrinogen and thrombospondin have been known to include RGD sequence. Also, some virus and some bacterium have been known to possess RGD sequence to adhere to cells. OPN is an acidic glycoprotein with binding properties to calcium which is contained rich in bone. It is reported that OPN plays an important role in cell adhesion, cell migration, tumor formation, immune response and complement mediated cellular lysis. Outcomes of OPN knockout mice and anti-OPN neutralizing antibodies indicate that OPN relates to hepatitis, autoimmune disease (such as rheumatoid arthritis), and metastasis of cancer. It has been noted that an inhibitor of binding of ECM proteins to cells may be used for treating osteoporosis or cancer. Thus, in addition to the above-mentioned antagonistic drugs targeted to integrins, antagonistic drugs targeted to the ECM proteins which are binding partner of the integrins have been developed.